Champaneria et al. U.S. Pat. No. 3,745,061 discloses synthetic filaments having 3 or 4 axially extending non-circular (in cross-section) voids. The 3 void version disclosed is nearly circular in cross-section.
Payne et al. U.S. Pat. No. 4,279,053 discloses triangular paint brush bristles having 3 axially extending non-circular voids. The voids are in the apexes of the angles.